In inhalation therapy, it is considered essential that the stream of gas, typically oxygen or air, be moisturized by the introduction of water vapor or nebulized particles of water prior to inhalation by a patient. Moisturization of the gas stream is usually accomplished by bubbling the gas through a bath of water in a humidifier, or by nebulizing the water into extremely fine particles which are then transported by the gas into the lungs of the patient. In order to heat the gas stream to body temperature for the comfort of the patient and to aid in the vaporization of the water, it is customary to heat the water bath to the desired operating temperature, either indirectly by the application of heat to the container holding the water or directly by inserting a suitable heating element into the water bath. The time required to heat the entire water bath to operating temperature, however, entails an undesirable delay before the apparatus is ready for use, while the direct introduction of a heating element into the water creates the possibility of producing an unsterile condition. Moreover, if the apparatus is not disposable, use by different patients also creates the risk of transfer of infection.